Add offset period aggregation type

[kael89]

Issue #:

[No issue] Required for PSSS indicators (eg https://github.com/beyondessential/tupaia-backlog/issues/1338), in order to calculate values for previous weeks (-1, -2 and -3 weeks) in comparison to the selected week

[edmofro]

Wonderful implementation and tests

One thing to consider: we have a very similar config for specifying the default start and end dates of a dashboard report, but it uses unit rather than periodType, e.g.

"defaultTimePeriod": {"unit": "month", "offset": -1}

periodType is certainly more in keeping with the rest of the aggregation package, so I'm fine with the difference here, but just wanted to raise it.

[kael89]

Wonderful implementation and tests

One thing to consider: we have a very similar config for specifying the default start and end dates of a dashboard report, but it uses unit rather than periodType, e.g.

"defaultTimePeriod": {"unit": "month", "offset": -1}

periodType is certainly more in keeping with the rest of the aggregation package, so I'm fine with the difference here, but just wanted to raise it.

Thanks @edmofro ! Very good point. I'd be keen on using the same key on both instances, so let's keep this in mind for a potential future renaming

By the way, could you please review the commit I just pushed (I forgot to include a util introduced in another PR)?